This invention pertains to a surgical implant and method to stabilize equine fetlock joints with a bone pin-plate that inserts into the third metacarpal bone and attaches to the tension surface of the first phalanx.
Horses, like humans, are susceptible to injuries sustained through physical exercise. The most common injuries sustained by horses are bone and joint fractures. While horses are usually capable of a complete recovery from minor bone fractures, a breakdown fracture in the ankle area, referred to as the fetlock joint or metacarpophalangeal joint, is usually catastrophic. Breakdown fractures can occur when the supporting structures of the fetlock joint give way under stress. Without the use of this supporting structure, a horse cannot satisfactorily support its own body weight.
Horses with catastrophic injuries to the fetlock supporting structure usually do not have an athletic future. Therefore, the goal of treatment is usually to salvage a horse for breeding by stabilizing the fetlock joint through arthrodesis. xe2x80x9cArthrodesisxe2x80x9d is the surgically-induced fusion of the bones of a joint. Complications often follow arthrodesis, including support limb laminitis, infection, implant failure, and cast sores. Such complications often mean that the treatment is ultimately unsuccessful.
The angles of the fetlock joint and the position of surrounding soft tissue make surgical stabilization of the fetlock supporting structure difficult. Surgical fixation with a bone plate is a method used for stabilization. However, the bone plate usually experiences fatigue failure due to cycling when placed on the compression side (dorsal side) of the third metacarpal bone, because the plate acts as the load carrying member. See M. Muller et al., Manual of Internal Fixation: Techniques Recommended by the AO Group, (2nd ed., Springer-Verlag, 1979) pp. 42, 58. As illustrated in FIG. 2, the compression side 28 of the third metacarpal bone is the side that experiences compression when weight is applied to the fetlock joint.
Unfortunately, the tension band side (palmar surface) of the horse""s fetlock joint lies beneath important supporting soft tissues. As illustrated in FIG. 2, the tension band side 26 of the fetlock joint 10 is the side that experiences expansion as weight is applied to the fetlock joint 10. See R. Kainer, xe2x80x9cFunctional Anatomy of Equine Locomotor Organs.xe2x80x9d in: Stashak, T.S., Adams"" Lameness in Horses (Philadelphia, Lea and Febiger, 1987), pp. 10-18. Equine surgeons have generally considered surgical approaches through these soft tissue structures too risky. Rather than risk damage to the soft tissues, surgeons have avoided the tension band side by placing the bone plate on the compression surface because the dorsal side is easier to access.
To minimize cyclic fatigue of the bone plate, most surgeons arthrodese (fuse) the fetlock joint into an unnatural straight line, resulting in an extended limb length. Horses usually adapt to the fusing of their fetlock joints in this unnatural position. However, unnatural fusion of the joints often causes a horse to overload its pastern and coffin joints because of uneven leg lengths, eventually causing secondary degenerative joint disease and pastern joint subluxation. See L. Bramlage, xe2x80x9cAn Initial Report on a Surgical Technique for Arthrodesis of the Metacarpophalangeal Joint in the Horse,xe2x80x9d Proceedings of the American Association of Equine Practitioners, vol. 27, pp. 257-261 (1982); and G. Crawley et al., xe2x80x9cA Modified Cloward""s Technique For Arthrodesis of the Normal Metacarpophalangeal Joint in the Horse,xe2x80x9d Veterinary Surgery, vol. 17, pp. 117-127 (1988).
Currently, there are four fundamentally different surgical devices to arthrodese horses"" fetlock joints. FIG. 1 illustrates a front plan view of one embodiment of an external skeletal fixation device. The device comprises transfixation pins 2, an external frame having a foot plate 4, and ascending vertical bars 6 placed on both sides of the third metacarpal bone 14. The transfixation pins 2 are inserted through the third metacarpal bone 14 and attached to the vertical bars 6. The foot plate 4 is attached to the hoof 8. However, this external fixation device is prone to infections and stress concentration around the pin-bone interfaces. See D. Richardson et al., xe2x80x9cUse of an External Skeletal Fixation Device and Bone Graft For Arthrodesis of the Metacarpophalangeal Joint in Horses,xe2x80x9d Journal of the American Veterinary Medical Association, vol. 191, pp. 316-321 (1987).
FIG. 2 illustrates a perspective view of one embodiment of a modified Cloward device. Two holes are drilled through the fetlock joint 10 and a steel basket 12, packed with autogenous porous bone, is driven into each hole. However, the stainless steel baskets 12 can induce stresses and have caused fractures emanating from the baskets into the proximal phalanx and the third metacarpal bone. See Crawley et al., 1988.
FIG. 3 illustrates a perspective view of one embodiment of an interlocking intramedullary nail device. A hole is drilled through the third carpal bone (not shown) into the medullary canal of the third metacarpal bone 14 and the first phalanx 16. An interlocking nail 18 is inserted through the third carpal bone, the metacarpal bone 14, and the first phalanx 16. Screws 20 are inserted through the interlocking nail 18, along the longitudinal axis of the first phalanx 16 and the metacarpal bone 14. See D. Herthel, xe2x80x9cApplication of the Interlocking Intramedullary Nail.xe2x80x9d in: Nixon, A. J., Equine Fracture Repair, (Pennsylvania, W. B. Saunders Company, 1996), pp. 371-376. However, clinical experience has shown that the interlocking nail method requires the fetlock joint be placed in an unnatural, completely straight position.
The most commonly used device today is the plate and tension-band wire device. FIG. 4 illustrates a side plan view of one embodiment of a plate and tension-band wire device. A plate 22 having one end contoured to fit the top surface of the first phalanx 16 is attached to the first phalanx 16 using screws 20, while the second end extends along the longitudinal axis of the third metacarpal bone 14. A figure eight tension-band wire 24 is placed on the tension surface through holes drilled transversely through the first phalanx 16 and third metacarpus 14 to absorb some of the cyclic load. After tightening the tension-band wire 24, screws 20 are inserted through the plate 22 and into the third metacarpal bone 14 to secure the fetlock joint 10. See H. Valdez et al., xe2x80x9cArthrodesis of the Fetlock Joint With Dynamic Compression Plates,xe2x80x9d Journal of Equine Medicine and Surgery, vol.3, pp. 421-427 (1979); and L. Bramlage, xe2x80x9cFetlock Arthrodesis.xe2x80x9d in: Nixon, A. J., Equine Fracture Repair, (Pennsylvania, W.B. Saunders Company, 1996), pp. 172-178. Nevertheless, a significant number of patients must be euthanatized (19 out of 43 cases in one study), usually because of support limb laminitis. See L. Bramlage, xe2x80x9cArthrodesis of the Metacarpophalangeal Jointxe2x80x94Results in 43 Horses,(Abstract)xe2x80x9d Veterinary Surgery, vol. 14, p. 49 (1985).
Support limb laminitis results from persistent, severe lameness arising from the inability of a horse to support all of its body weight on one leg. Asymmetric limb loading causes mechanical failure of the attachments between the hoof and the bone in the hoof. Horses treated with the tension-band wire technique will likely remain lame because the fixation is not optimally stable. Unstable fixations result in movement between the bone ends, causing pain, slower healing, and prolonged unilateral weight bearing. See Muller et al., 1979. All of these factors increase the probability of support limb laminitis.
I have discovered a bone pin-plate and a surgical method that promote bone fusion of a fetlock joint breakdown by supporting and stabilizing the limb. The device is a bone pin-plate combination capable of being implanted and attached to the tension band surface (rear surface) of the first phalanx. The device comprises a proximal end having a cylindrically-shaped pin, and a distal end having a plate extending at an angle away from the longitudinal axis of the pin. The cylindrically-shaped pin is adapted to be inserted through the center of the third metacarpal bone. The pin merges with a plate upon exiting the third metacarpal bone, preferably as an integral piece.
The bone pin-plate incurs a tension force when weight is applied to the fetlock joint, whereas prior devices incur a compression force, which tends to cause cycling, eventually resulting in mechanical failure. The bone pin-plate""s rigid construction substantially eliminates cycling. The bone pin-plate is inserted through the center of the third metacarpal bone, and thus evenly distributes weight along the length of the bone. Additionally, the bone pin-plate allows a surgeon to support and stabilize a fractured fetlock joint in a natural position.
The invention allows the relatively inexpensive, stable fixation of an equine fetlock joint after a fetlock joint fracture, through the tension band surface of the first phalanx. Because the novel bone pin-plate uses the tension band surface of the first phalanx, it reduces mechanical failure, unilateral weight bearing, and pain. Thus, the bone pin-plate promotes an increased healing rate of an equine fetlock fracture.